Butter knife



FIG. 1 is a top, right side perspective view of a butter knife showing my new design, the left side being a mirror image of the right;

FIG. 2 is a right side elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a front elevational view thereof; and

FIG. 6 is a rear elevational view thereof. 

The ornamental design for a butter knife, as shown and described. 